After fretting for months that a “critical part” of the world’s biggest bunker-busting bomb was flawed, the Pentagon declared last week that the mammoth blaster is at last fixed and ready for use – against Iran, or any other nation the U.S. decides is developing weapons of mass destruction deeply underground.

There’s as much psychological gamesmanship at play here as pyrotechnics, as the U.S. and Iran continue their showdown over Washington’s insistence that Tehran abandon its nuclear-weapons program, and the Iranians’ assertion that their program is only for peaceful purposes.

Make no mistake about it: the declaration that the Air Force’s Massive Ordnance Penetrator is ready for action is the latest volley in a high-stakes game that could lead to war before the year is out.

Air Force photo

A fake MOP in the fake belly of a fake B-2 bomber.

The Defense Department’s Operational Test and Evaluation office makes clear that, after more than a year of vague Pentagon-expressed concerns that there were technical problems with the MOP, the 15-ton bomb is now ready for prime time. The green light comes after a series of tests through the fall at Holloman Air Force Base and White Sands Missile Range, both in New Mexico, showed that the redesigned weapon “effectively prosecuted the targets.”

“The MOP is a GPS-guided weapon designed to reach and destroy targets located in well protected facilities,” the testing synopsis reads. “The sled test results and the additional weapon drops indicate that the weapon re-design is adequate for the successful prosecution of all of the elements of the currently defined target set.”

While no one will say so, Iran’s nuclear facility at Fordo – buried up to 80 meters beneath a mountain near the Shiite holy city of Qom, at a former missile base controlled by Iran’s unpredictable Revolutionary Guards – is at the top of that target list. And Tehran now knows it is officially vulnerable to attack.

“The warhead case is made from a special high performance steel alloy and its design allows for a large explosive payload while maintaining the integrity of the penetrator case during impact,” the test report notes. “Combatant Commanders use MOP to conduct pre-planned, day or night attacks against defended point targets vulnerable to blast and fragmentation effects and requiring significant penetration, such as hardened and deeply-buried facilities.”

Having the MOP in its quiver does two things for the U.S.: first of all, for the Iranians, it makes clear that the U.S. has a non-nuclear option to pulverize much of its nuclear-development effort – peaceful or otherwise – and delay it for years. But the neat twist is that Israel doesn’t have such a weapon, and its relatively puny 5,000-pound bunker-busters could do far less damage to Iranian targets. That, some U.S. officials say, should act as a brake on any unilateral Israeli action.

Nearly a decade ago, the Pentagon concluded that dropping its one-ton bombs on buried targets was like using a peashooter against an elephant. “Our past test experience has shown that 2,000-pound penetrators carrying 500 pounds of high explosive are relatively ineffective against tunnels, even when skipped directly into the tunnel entrance,” a 2004 report said. “Instead, several thousand pounds of high explosives coupled to the tunnel are needed to blow down blast doors and propagate a lethal air blast throughout a typical tunnel complex.”

Tehran acknowledged Fordo’s existence in 2009, once it learned the West had detected it and was ready to announce its discovery. That was about the same time the Pentagon’s Defense Threat Reduction Agency — charged with reducing the threat posed by weapons of mass destruction – was wrapping up initial tests on the MOP’s design before handing the program over to the Air Force.

The Air Force said it wanted a “Quick Reaction Capability” to “defeat a specific set of Hard and/or Deeply Buried Targets.” The weapon, the service said, would “maximize effects against Hard and/or Deeply Buried Targets (HDBTs), while minimizing time over target.” The service said it needed the weapon to meet “Urgent Operational Needs requirements” – generally a plea from a battlefield commander who doesn’t think he has the weapons he needs to accomplish a mission assigned to him.

“The system will hold at risk those highest priority assets essential to the enemy’s war-fighting ability, which are heavily defended and protected,” the Air Force said in February 2011 budget documents. It would provide “a critical global strike capability not currently met by inventory conventional weapons.”

The $15 million MOP has six times the heft of existing GBU-28 bunker busters. Glided into its destination by GPS-guided lattice-type fins, its alloy steel hull – some 80% of its weight – is designed to remain intact as it drills through rock or reinforced concrete before setting off its 5,300-pound warhead. Air Force officials say it represents a “bridge” capability between existing bunker busters and nuclear weapons themselves.

Pentagon documents detail the kind of target Fordo is believed to represent (“all data with regard to this Scenario are notional,” the government says, “and do not represent any known target”):

…the HDBT is assumed to be a tunnel complex with two portals built into the base of a granite mountain. The granite over layer is assumed to be 60 meters thick over both portals and 80 meters over the facility’s mission space. Metallic blast doors are closed and hermetically sealed at both portals. The two tunnels are assumed to be 100 feet in length with directional turns. The facility is constructed on a single level and occupies between 10,000 and 20,000 square feet.

The HDBT makes use of state-of-the-art computers and communication equipment. It is tied to the local electricity grid but has a large capacity diesel generator to produce its electricity when the need arises. The facility draws fresh water from an internal deep well. Sewage is held in holding tanks and removed periodically. All incoming air passes through state-of-the-art biological/chemical filters.

The Pentagon doesn’t have an unlimited supply of MOPs: it initially bought 20, for $314 million. The Boeing-built weapon can only be carried by the Air Force’s B-2 stealth bomber.

It’s also extremely shy: there are few photographs of the real thing. But the Air Force has released a pair of photographs of a MOP mockup. It’s a safe bet the mockup has been tweaked from the actual weapon to avoid betraying its precise design and dimensions.

Defense Secretary Leon Panetta was asked about the MOP’s possible use against Iranian targets in September. “Without going into the particular capabilities we have,” Panetta told CBS, “we think we’ve got the ability to be able to strike at them effectively if we have to.”